A virus-infected network of nearly 13 million computers around the world has been smashed by Spanish police.

The "botnet" included PCs inside more than half of America's 1,000 biggest companies and more than 40 major banks.

Botnets are networks of PCs that have been hijacked from their owners, often without their knowledge, and put into the control of criminals.

Linked together, the machines supply an enormous amount of computing power to spammers, identity thieves and internet attackers.

The affected computers stole credit card numbers and online banking credentials.

Police, working with private computer-security firms, arrested three young Spaniards last month as the alleged ringleaders of the Mariposa botnet, which appeared in December 2008 and grew into one of the biggest weapons of cybercrime.

The Spanish authorities are looking for a fourth suspect who might be Venezuelan.

The people in custody did not design the malicious software behind the grid; they just bought it on the black market, a police spokesman said.

"We have not arrested the creator of the botnet. We have arrested the administrators of the botnet, the ones who spread it and were administering and controlling it," he said.

He declined to say how much money might have been plundered or name companies whose computers had been compromised.

An estimated 4,000 to 6,000 botnets are operating today and this one was the biggest one ever brought down, said Jose Antonio Berrocal, head of the Spanish Civil Guard's economic and technological crimes unit.

The Mariposa botnet spread to more than 190 countries, according to researchers. It also appears to be far more sophisticated than the one that was used to hack into Google and other companies in the attack that led Google to threaten to pull out of China.